The present invention relates to devices and methods for measuring currents.
Fiber optic current sensors based on the Faraday effect are attractive for remotely measuring large electrical current, because they have numerous advantages over traditional inductive current transformers. These advantages include wide dynamic range, fast response, immunity to electromagnetic interference, small size, and low cost. Therefore, a variety of fiber optic current sensors have been investigated over the last few years, mainly using silica fibers.
These sensors have not yet reached the stage of practical field use due to lack of accuracy and stability. This is mainly because the Faraday rotation is quenched by intrinsic and induced linear birefringences. These birefringences are easily generated in the silica glass fibers used as a sensing element. Further, these fibers have not been able to accurately measure large currents, such as surge or fault currents.
The Faraday effect is a phenomenon by which a linear, polarized light will rotate when propagating through a transparent material that is placed in a magnetic field in parallel to the magnetic field. The size of the rotation angle (e), given in degrees, is defined as
xcex8=VHLxe2x80x83xe2x80x83(1)
where H is the strength of the magnetic field (A/m), V is the Verdet constant of the material, and L is the path length over which the magnetic field acts (m).
The magnetic field strength is measured in terms of Amperes (A) times turns (T) per unit length (AT/m) where m is meters). Since values are expressed in terms of one turn, this factor is usually implicit, rather than explicit. Hence, the strength is customarily given in amperes (A) or kiloamperes (kA) per unit path length in meters (m).
The Verdet constant, V, is the angle of rotation divided by the magnetic field strength per unit length. The angle may be expressed in any of the customary units for angle measurement, but degrees are used here. Verdet constant values, unless otherwise indicated, are given in terms of degrees divided by field strength expressed as (kAxc3x97T/m)m.
The magnitude of the magnetic induction (B) around an infinite straight conductor is given by the expression:
B=(xcexco/4xcfx80)(2I/a)xe2x80x83xe2x80x83(2)
where I is the current, p is permitivity of free space and a is the radial distance of the magnetic field from the conductor. The magnetic field is related to the magnetic induction by the simple relation:
B=xcexcoH.xe2x80x83xe2x80x83(3)
Combining equations 1 through 3 gives the desired relation between the rotation and the current:
xcex8=VIxe2x80x83xe2x80x83(4)
where xcex8 is in degrees, V is the Verdet constant, and I is in kA. Thus, the sensitivity of a method for measuring the current depends on how accurately one can measure the angular rotation.
The degree of sensitivity in measuring the angular rotation is influenced by another factor; birefringence. Birefringence arises primarily from stresses that result from bending, or otherwise distorting, the fiber in its disposition. The sources of linear birefringence in single mode fibers include residual stress from fabrication, bending, contact, and thermal stresses (Yamashita et al., xe2x80x9cExtremely Small Stress-optic Coefficient Glass Single Mode Fibers For Current Sensorxe2x80x9d, Optical Fiber Sensors, Sapporo Japan, paper We2-4, page 168 (1996) (xe2x80x9cYamashitaxe2x80x9d).
Accordingly, one important feature of a device used to measure current is a single mode optical fiber (xe2x80x9cSMFxe2x80x9d) so that the path can be maintained with a large Faraday effect (i.e. high Verdet constant) and no birefringence.
A glass composition in which no birefringence is produced by any applied stresses is a glass that produces a zero value of the photoelastic effect. The stress-induced birefringence is quantified in terms of a coefficient, called the photoelastic constant (or the photoelastic coefficient). The photoelastic coefficient (Bp) may be defined as the coefficient relating the difference in the refractive indices in the stress direction (n(par)) and in the perpendicular direction (n(per)), to the magnitude of the applied stress:
n(par)xe2x88x92n(per)=Bp"sgr"xe2x80x83xe2x80x83(5)
It may also be regarded as the phase shift measured in units of wavelength in nanometers (nm) per path length in centimeters (cm) divided by the stress in kilograms per square centimeter (kg/cm2). The values then are in units of (nm/cm divided by kg/cm2).
In some cases, it is also desirable to have a device that measures surge currents via the Faraday effect. This is important, because surge currents, which are unexpected large currents, will cause large angles of rotation. To measure surge currents, it is important to keep the angle of rotation below 90 degrees. With glasses having larger Verdet constants, when a fault current occurs, angles of rotation of greater than 90 degrees would result. These angles of rotation greater than 90 degrees would register the same as an angle of less than 90 degrees. In contrast, a device having a fiber made of a glass having a lower Verdet constant would not have as great an angle of rotation on a fault current. Therefore, it would accurately measure such currents.
Japanese Patent Application No 3-13177 relates to a glass fiber for devices for magnetic field current measurements. The glass has a large Verdet constant and a small photoelastic coefficient. The glass has a core and clad material having the following composition ranges by weight %: 5-28 SiO2, 0-10 B2O3, 0-5 Al2O3, 16-28 SiO2+B2O3+Al2O3, 0.3-2.5 Na2O+K2O, and 69.5-83.7 PbO.
This glass requires large amounts of lead to produce a zero photoelastic coefficient. Further, the glass of the Japanese application is not suitable for use in measuring surge currents. There, a glass having a smaller Verdet constant, but still having a photoelastic coefficient equal to or approaching zero, is required.
The present invention is directed to providing a current measuring device that overcomes these deficiencies.
The present invention relates to a device for measuring a current in a magnetic field comprising a glass article having a photoelastic coefficient of from about xe2x88x920.2 to about 0.2 at 546 nm, the glass being selected from a group of glass families consisting of oxyfluoride and bismuth oxide-containing glasses.
Another aspect of the invention is a method of measuring a current in a magnetic field which comprises providing, as a current sensor, a glass article that has a photoelastic coefficient ranging between about xe2x88x920.2 to about 0.2 at 546 nm, and that is capable of causing an angular rotation of polarized light propagated through the glass article, the glass being selected from a group of glass families consisting of oxyfluoride and bismuth oxide-containing glasses. The method further comprises passing a current through a conductor which creates a magnetic field surrounding the conductor. The current flows through the current sensor, propagating a polarized light into the glass article. The angle of rotation of the polarized light in the glass article is then determined so that the current can be measured.
The present invention advantageously provides a device having a glass article which has a large Verdet constant and a photoelastic coefficient suitable for measuring electrical currents. In addition, the present invention provides a device which can be used to accurately measure surge currents.